(1) Field of the Invention
The present invention relates to a semiconductor integrated circuit (IC), more particularly to an IC in which a status of a node in an internal circuit thereof can be inspected through an external input/output pin thereof.
(2) Description of the Prior Art
The increasingly higher densities of packing of IC's have led to the evolution of large-scale integrated circuits (LSI) and very large-scale integrated circuits (VLSI). LSI's and VLSI's contain enormously large numbers of circuits. Accordingly, there is an extremely high probability that several defective circuit elements are created during the wafer processes. Naturally, it is not desirable, in view of the wafer process yield, to discard an entire chip containing, for example, several hundred thousand circuit elements, due to the creation of a handlful of defective circuit elements.
To improve the yield, manufacturers usually fabricate a redundancy circuit in advance in the chip. Therefore, if they find defective circuit elements during their probing tests of the chip, they use the redundancy circuit to function for the defective circuit elements. This enables a remarkable improvement in the manufacturing yield of the chip. This method is widely used for a variety of IC products, such as semiconductor memory devices.
The introduction of a redundancy circuit into a chip enables large numbers of IC's which might otherwise have been discarded to be saved, even though they may contain one or more defective circuit elements. On the other hand, IC's with redundancy circuits suffer from certain problems. That is, after the IC chips are completely packaged, it is impossible to determine whether the redundancy circuit has been used or is still unused. It is, of course, possible to determine this by disassembling the IC's, however, such disassembled IC's could not be used commercially.
IC manufacturers must be able to know whether packaged IC's contain a used redundancy circuit or an unused redundancy circuit in order to cope with complaints from the user. For example, in random write/read operation tests for IC memory devices, the results of the tests may differ according to whether a circuit element (for example a memory cell), specified by certain address input, is located at its originally intended position or has been replaced by a redundancy circuit.
One proposal to solve this problem calls for the use of an idle external input/output pin, electrically connected to the circuit elements, for discriminating whether the IC contains a used or unused redundancy circuit. In actuality, however, and particularly in the case of an LSI or VLSI, there is almost never an idle external input/output pin. Consequently, the proposal is not practical.